Mean Sac Diameter Calculator

mm
mm
mm
Health

The Mean Sac Diameter Calculator averages the three ultrasound dimensions of a gestational sac and gives a rough gestational age estimate. Enter the length, width and height in millimetres.

Formula

MSD = (Length + Width + Height) ÷ 3; GA (days) ≈ MSD (mm) + 30
  • The gestational age rule of thumb (MSD + 30 days) is a rough early-pregnancy estimate.
  • Crown-rump length is more accurate once an embryo is visible.

20 × 18 × 16 mm sac

Inputs
  • Length: 20 mm
  • Width: 18 mm
  • Height: 16 mm

(20 + 18 + 16) ÷ 3 = 18 mm; GA ≈ 48 days ≈ 6.9 weeks.

Frequently asked questions

How accurate is MSD for dating?
It is a rough estimate used before an embryo is visible. Crown-rump length is preferred once measurable.